[RFC PATCH 20/76] mm: Stop generic_file_buffered_read() from grabbing a superfluous page

[David Howells]

Under some circumstances, generic_file_buffered_read() will allocate
sufficient pages to read to the end of the file, call readahead/readpages
on them and copy the data over - and then it will allocate another page at
the EOF and call readpage on that and then ignore it.  This is unnecessary
and a waste of time and resources.

Catch the overallocation in the "no_cached_page:" part and prevent it from
happening.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 mm/filemap.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index cfb753955e36..5a63aa1dd71e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2447,6 +2447,8 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 		 * Ok, it wasn't cached, so we need to create a new
 		 * page..
 		 */
+		if ((index << PAGE_SHIFT) >= i_size_read(inode))
+			goto out;
 		page = page_cache_alloc(mapping);
 		if (!page) {
 			error = -ENOMEM;